1. Field of the Invention
This invention relates to a semiconductor laser device possessing a refractive index waveguide mechanism which is capable of operating at high output and at low threshold current.
2. Description of the Prior Art
For a semiconductor laser used in an optical information processing apparatus such as a light signal and an optical disc player, it is indispensable that the laser possess a refractive index waveguide mechanism. Such a semiconductor laser was, conventionally, fabricated by a liquid phase growth process, and various structures were proposed for incorporating said refractive index waveguide mechanism. For example, in the construction using GaAlAs compound as a material, the VSIS (V-channeled substrate inner stripe) laser with the substrate processed in a groove form, CSP (channeled substrate planar) laser, and BH (buried heterostructure) laser having the light-emitting region buried with clad layer are known.
In these laser devices, however, since the liquid phase growth process is used in their manufacture, it is difficult to control the film thickness or composition of each layer that makes up the laser device, and it is also difficult to form very thin active layers in a clean shape free from lattice defects, and it is hence extremely difficult to realize a high performance semiconductor laser low in threshold current density. Besides, the manufacturing yield is poor. To overcome these disadvantages of the liquid phase growth process, and so as to provide a crystal growth method which controls the film thickness very strictly, a molecular beam epitaxial (MBE) process controlling the vacuum deposition technology with extremely high precision, and metalorganic chemical vapor deposition are being introduced recently. Particularly, in the MBE process, it is possible to control the film thickness on the order of the atomic layer, using vacuum analyzers and electronic computers. In this method, however, since the crystal growth mechanism of the semiconductor laser is different from that of the liquid phase growth process, the device structure possessing various refractive index waveguide mechanisms proposed in the liquid phase growth process cannot be applied in production, and other new device structures must be employed. As such new device structures, for example, the ridge waveguide type semi-conductor laser shown in FIG. 1 and semiconductor laser device shown in FIG. 2 are proposed.
In the ridge waveguide type laser device in FIG. 1, n-type GaAlAs clad layer 32, GaAs active layer 33, p-type GaAlAs clad layer 34, and p-type GaAs cap layer 37 are sequentially formed on a flat n-type GaAs substrate 31, and all of the sides of cap layer 37 and part of the sides of clad layer 34 are removed by etching. SiO.sub.2 insulation layer 36 is formed on the surface of both the sides of this clad layer 34, and finally a p-side electrode 38 and an n-type electrode 39 are respectively formed on the upper surface and lower surface of the convex form. In the semiconductor laser in FIG. 2, which is manufactured by metalorganic chemical vapor deposition, n-type GaAlAs clad layer 22, GaAs active layer 23, p-type GaAlAs clad layer 24, and n-type GaAs current narrowing layer 25 are sequentially formed on a flat n-type GaAs substrate 21, and the middle part of this current narrowing layer 25 is removed in stripes by etching, and p-type GaAlAs clad layer 26 and p-type GaAs cap layer 27 are formed thereon. Finally, a p-type electrode 28 and an n-type electrode 29 are formed on the upper surface and lower surface of the concave form, respectively.
In the laser device in FIG. 2, since it is taken out of a quartz reaction tube or the like for etching, the surface of the n-type GaAs current narrowing layer 25 is exposed to the atmosphere to be oxidized. In particular, since the surface 20 is close to the light-emitting region 23a of active layer 23, and device deterioration derived from oxygen is likely to occur, and moreover since the current narrowing layer 25 is made of GaAs, light absorption occurs in this area, and it is not desired for reduction of threshold current density. Besides, when GaAlAs is used, for example, as the current narrowing layer 25, crystal growth on the surface is extremely difficult due to oxidation of the GaAlAs surface at the time of etching.
Therefore, in the structure in FIG. 2, it is difficult to sufficiently utilize the low threshold current density characteristics of the MQW (multi quantum well) laser and GRIN-SCH laser making use of the excellent film thickness characteristics and controllability of the composition of the MBE process. On the other hand, the ridge waveguide type semiconductor laser is hard to handle because ridges are exposed on the surface, and it is difficult to mount on the growth layer side where the oscillation region is present. If it is mounted by setting the growth layer up, cooling performance is poor, and it causes problems in the aspects of device reliability and high output operation. Therefore, as the laser device of ridge waveguide type, actually, it seems effective to form a supporting part of the same height as the ridge at both sides of the ridge as shown in FIG. 3. This laser device includes layers from the previously discussed devices, i.e. a substrate 41 on which clad layer 42, active layer 43, clad layer 44, current narrowing layer 45, and insulation layer 46 are formed. Electrodes 47 and 48 are formed on the insulation layer and substrate, respectively. But the following problems are involved with the structure in FIG. 3. That is, in the ridge waveguide type laser device, the device characteristic greatly depends on the ridge shape (ridge width, or thickness from active layer at both sides of ridge to the surface), and to control this ridge shape as accurately as possible, it is desired that the total thickness of the second clad layer 34 and cap layer 37 may be small to an extent that the device characteristics may not be worsened. In this case, however, when mounting the device, the solder such as In goes up along the element end surface to be adhered, and the possibility of short-circuiting the pn junction of device becomes very high.